Electrochemical cells (fuel cells, electrolysers, sensors etc.) are a key element in the development of a more efficient and environmentally-friendly utilisation of fossil fuels, and in the development of systems for renewable energy based on hydrogen or a hydrogen carrier as storage medium for direct or indirect solar energy. Such cells need an ionically conducting or ion-conducting membrane (i.e. an electrolyte). This may be liquid (as in the lead-acid battery) or solid.
Solid electrolytes have many practical advantages over liquid based electrolytes as they allow, inter alia, greater energy density. To date, solid electrolytes have, for the most part, been based on oxygen ion conducting solid oxides which work at a high temperature (>600° C.) or on proton conducting polymeric electrolytes which only work at low temperatures (<100° C.). Devices made with the former electrolytes suffer from short lifetimes because of the high temperatures at which they are employed, whilst the latter electrolytes have problems with water balance and catalyst poisoning because of the low temperatures employed. Moreover, at present both technologies are costly.
Proton conducting oxides have recently emerged as a possible alternative to the electrolytes above and have shown high ionic conductivity at intermediate temperatures. However, these proton conducting oxides employ alkaline earth metals as principal components and are thus basic, which renders their use with fossil fuel impossible because of reaction with CO2. There is thus a need to identify oxides which are less basic (e.g. which do not have alkaline or alkaline earth metals as principal components) and which have proton conductivity, and to demonstrate how this conductivity can be optimised and utilised.
Proton conductivity has previously been described at high and intermediate temperatures in a number of oxides and oxidic materials. As mentioned above, these include a number of Sr and Ba-containing oxides—primarily perovskites AMO3 where A is Sr, Ba; M is Zr, Ce, Th, Ca+Nb or Sr+Nb (See Kreuer, “Proton conductivity; materials and applications”, Chem. Mater., 8 (1996) 610-641). Among materials that do not have alkaline earth metals as principal component, proton conductivity has been described in rare earth sesquioxides Ln2O3 where Ln is Y, La—Lu (See Haugsrud et al, “Proton conductivity of Ca-doped Tb2O3”, submitted to Solid State Ionics), perovskites AMO3 where A is La; M is Sc, Y, Er (See Larring et al “Protons in rare earth oxides”, Solid State Ionics, 77 (1995) 147-51), pyrochlores La2Zr2O7 (See Shimura et al Solid State Ionics, 86-88 (1996) 685-8), and orthophosphates LnPO4 where Ln is La, Nd, Gd (See Norby et al, Solid State Ionics, 77 (1995) 240). However, the maximum proton conductivity has been more than an order of magnitude lower than that of the best Sr and Ba-containing materials, and some of the materials have limited stability at room temperature, others at operating conditions in fuel cells etc.
The object of the present invention is to provide new, improved materials which are suitable for use as proton conducting electrolytes, and which do not have the aforementioned disadvantages.
It has now surprisingly been found that doped rare earth orthoniobates and orthotantalates give an unexpectedly high proton conductivity and therefore are highly suitable as proton conducting electrolytes. This material group does not contain alkaline earth metals as principal components but are instead formed from a combination of a moderately basic rare earth oxide and an acidic Nb or Ta oxide, which gives good chemical stability. They have high melting points. Whilst certain doped orthoniobates and tantalates are known, e.g. as described in U.S. Pat. No. 6,620,750, never before have they been converted into proton conducting electrolytes.